The present invention relates to a protective layer transfer sheet which can form a protective layer by thermal transfer on an object, such as an image-receiving sheet with an image formed thereon. More particularly, the present invention relates to a protective layer transfer sheet which, when used to transfer a protective layer onto a print, can impart excellent fastness properties to the print.
At the present time, thermal transfer recording is widely used as a simple printing method. The thermal transfer recording can simply form various images, and thus is utilized in printing wherein the number of prints may be relatively small, for example, the preparation of ID cards, such as identification cards, photographs for business, or printers of personal computers or video printers. When a full-color halftone image, such as a photograph-like image of a face, is preferred, the thermal transfer sheet used is such that, for example, various colorant layers of yellow, magenta, and cyan (and, in addition, optionally black) are provided as ink layers in a large number in a face serial manner on a continuous substrate film. Such thermal transfer sheets are classified roughly into thermal transfer sheets of the so-called xe2x80x9cmelt transfer (ink transfer)xe2x80x9d type wherein the colorant layer is melted and softened upon heating and as such is transferred onto an object, that is, an image-receiving sheet, and thermal transfer sheets of the so-called xe2x80x9csublimation (dye transfer)xe2x80x9d type wherein, upon heating, a dye contained in the colorant layer is sublimated to permit the dye to migrate onto the image-receiving sheet.
When the above thermal transfer sheet is used, for example, to prepare ID cards, such as identification cards, the melt transfer type is advantageous in that line images, such as letters or numeric characters, can be easily formed, but on the other hand, the fastness properties, particularly abrasion resistance, of the formed images are disadvantageously poor. On the other hand, the sublimation type is suitable for the formation of halftone images, such as photograph-like images of a face. Unlike conventional printing inks, however, no vehicle is used. Therefore, the formed images are poor in fastness properties such as abrasion resistance, and, in addition, when brought into contact, for example, with plasticizer-containing card cases, file sheets, erasers made of plastics or the like, disadvantageously cause migration of dyes onto them. Further, the formed images have poor chemical resistance, solvent resistance and other properties and hence cause blurring or other unfavorable phenomena. For this reason, an attempt to further transfer a protective layer on the formed image from a protective layer transfer sheet has been made to further impart improved fastness properties, such as abrasion resistance, chemical resistance, and solvent resistance, to the formed images. For example, a protective layer transfer sheet comprising a substrate film, a transparent resin layer releasably provided on the substrate film, and a heat-sensitive adhesive layer provided on the transparent resin layer is used to transfer and stack a transparent resin layer on an object with an image formed thereon through the heat-sensitive adhesive layer.
The image-receiving face with an image being transferred by sublimation dye transfer is highly releasable from the viewpoint of preventing heat fusing between the image-receiving face and the dye in the thermal transfer sheet, but on the other hand, the adhesion is low. Therefore, in the conventional protective layer transfer sheet, the adhesion of the protective layer to the image-receiving sheet is unsatisfactory, and, thus, when the print is stored under some conditions, the transferred protective layer is disadvantageously cracked.
Further, when a print comprising a protective layer thermally transferred onto an image-receiving sheet with an image formed thereon is housed and stored in a vinyl chloride case, a dye constituting the image in the print migrates to the case and this disadvantageously poses problems such as lowered image density of the print.
Accordingly, it is an object of the present invention to provide a protective layer transfer sheet, comprising a substrate sheet and a thermally transferable protective layer releasably provided on the substrate sheet, which, when used to thermally transfer a protective layer onto an image-receiving sheet with an image formed thereon, can provide a print that does not cause cracking of the protective layer and, even when housed and stored in a vinyl chloride case containing a plasticizer or the like, does not cause the migration of a dye constituting the image to the case, that is, has excellent fastness properties.
According to the present invention, there is provided a protective layer transfer sheet comprising: a substrate sheet; and a thermally transferable protective layer releasably provided on the substrate sheet, said thermally transferable protective layer containing an epoxy rein and, in addition, containing at least one member selected from the group consisting of a butyral resin, an acrylic resin, an ultraviolet absorber, and a mixture thereof.
Thus, in this protective layer transfer sheet according to the present invention, since the thermally transferable protective layer contains, in addition to an epoxy resin, at least one member selected from the group consisting of a butyral resin, an acrylic resin, an ultraviolet absorber, and a mixture thereof, the protective layer is strongly adhered onto an image-receiving sheet with an image formed thereon. Further, the protective layer is strong and homogeneous, and, thus, even when the print is housed in a vinyl chloride case, the print does not cause the dye to migrate to the case, that is, has excellent plasticizer resistance.
According to a preferred embodiment of the present invention, the thermally transferable protective layer comprises a combination of a release layer, a main protective layer, and an adhesive layer provided in that order from the substrate sheet side, and the adhesive layer contains an epoxy resin and, in addition, contains at least one member selected from the group consisting of a butyral resin, an acrylic resin, and an ultraviolet absorber. Also in this embodiment, since the adhesive layer, which comes into contact with the image-receiving sheet upon the transfer of a protective layer onto the image-receiving sheet, contains an epoxy resin and, in addition, contains at least one member selected from the group consisting of a butyral resin, an acrylic resin, and an ultraviolet absorber, the adhesive layer is strongly adhered onto the image-receiving sheet with an image formed thereon. Further, in particular, the adhesive layer is strong and homogeneous, and, thus, even when the print is housed in a vinyl chloride case, the print does not cause the dye to migrate to the case, that is, has excellent plasticizer resistance.
According to a preferred embodiment of the present invention, the release layer contains at least an acrylic resin.
Further, in the present invention, preferably, the release layer is non-transferable, and, upon thermal transfer, the release layer stays on the substrate sheet while the protective layer is separable from the substrate sheet.
According to another embodiment of the present invention, upon thermal transfer, the thermally transferable protective layer may be separable directly from the substrate sheet.
Further, preferably, the epoxy resin has a molecular weight in the range of 800 to 6000 and a glass transition temperature of 60 to 150xc2x0 C.